Known from practical use is a method of the type defined above for making tubes to be employed in exhaust mufflers. A whole pattern desired for the finished tubes is punched into a flat metal strip. The strip is subsequently formed, as by a roll-forming method, to a tubular shape, its longitudinal edges being connected by a weld seam. Subsequent to the welding step, the tubes are cut to the desired length by means of a cutting apparatus, the operation of which is dependent on the hole pattern.
This known method suffers from certain drawbacks. As the introduction of the welding energy requires a continuous metal strip to be present along both sides of the weld seam, the rows of holes cannot be evenly distributed around the periphery.
The coordination of the various production steps likewise offers certain difficulties. The welding step requires a minimum feed velocity which is too high with regard to the hole formation in the metal strip by means of conventional punching presses. For this reason the metal strip is usually unwound from a coil, punched and rewound in a separate operation. As the hole patterns may vary with the type of tubes to be made, the punching operation can only be carried out in coordination with the type of tubes as ordered.
The accurate cutting of the tube to length depending on the hole pattern requires the employ of suitable scanning and aligning means.
The known method is thus relatively expensive and cumbersome and does not lend itself to the economical production of small numbers.
Known from U.S. Pat. No. 1,510,718 is an unrelated apparatus for forming holes in roller bearing cages. The cages are of frustoconical configuration and are to be formed with rectangular openings in their peripheral wall surface by a punching operation. The punching tool is inserted into the cage blanks and actuated in a radially outward direction, the outer periphery of the cage blanks being backed by a support element.
This known apparatus is only applicable in the case of workpieces suitable for being processed in a sideways direction and having an opening of a size permitting the tools for such processing to be inserted from the exterior into the interior of the workpiece, while the tool actuating mechanism remains substantially outside of the workpiece. The processing of the workpieces is additionally facilitated by the angular attitude of their peripheral wall with respect to their axis, as it permits the tools to be inserted throught the wider opening of the workpieces. The known apparatus is unsuitable, however, for forming holes in the peripheral wall of tubular workpieces having a substantially constant and relative narrow cross-section, as the tools can only brought to bear, if at all, adjacent the open ends of the tubular workpieces, but not in the axially intermediate region thereof.